Firmware D13
When the system boots up, it often will say Firmware D13 before resuming normal operation. If the system does not begin normal operation, then you likely have a connection issue. Contrary to what is displayed, firmware D13 is not a firmware, but an error message indicating a communication issue with the board itself OR a component plugged into the board.
A good start is to give your wiring a once over and see if something is be corroded, badly worn, or unplugged, as that would be the easiest and most likely solution.
If you don’t see anything obvious, then access your MPC board and unplug/unscrew everything besides the input voltage on the pegs, your display cable, and salinity probe (remove all other green pin plugs and any signal wires.)
Turn the system off and on again. If you still have the Firmware D13 message, than the issue is likely with your display, display cable or potentially the board itself.
Upon restart if the message is gone and you get something other than D13, turn the system off, add on one of the signal plugs or wires, and look again for the D13 error code. If it does not come on, then continue with each plug/wire until you find the one with the issues. Then you know what wires to chase - or to look at a wiring diagram to find out what sensor might be giving you the issue.
Many system wiring diagrams can be seen in their respective manuals at this link Spectra Manuals, MPC + Older Wiring Diagrams